Meniscus tissue is comprised of a type of tissue known as fibrocartilage. Fibrocartilage is present in the form of a disc (spine, temporo-mandibular joint), meniscus (knee), labrum (shoulder, hip), etc. In the knee, as shown in FIG. 1, the meniscus is a semi-lunar, wedge shaped tissue that sits on top of the tibia and articulates with the tibia and femur during gait activities. It acts as a shock absorber between the femur and tibia and distributes the compressive and shear loads from the curved condyles of the femur to the relatively flat plateau of the tibia. Similar to articular cartilage, much of the meniscus is avascular and aneural. However, as shown in FIG. 2, the meniscus has three zones of vascularity: red zone, red/white zone, and white zone. The red zone refers to approximately the outer peripheral third of the meniscus. This zone is rich in blood supply. The white zone can be found in the approximate inner peripheral third of the meniscus and is void of blood supply, and the red/white zone can be found in the approximate middle third and has a limited blood supply.
Injuries and pathologies occur in the meniscus, labrum, and disc that manifest themselves in the forms of tears, as shown in FIG. 3, degeneration or other tissue defects. For the purposes of this document, “tissue defect” means any form of non-healthy tissue including, but not limited to, tissue tears, degenerative tissue, non-union bone fracture defects, or any other form of non-healthy tissue. Various types and degrees of tears and defects in the knee meniscus can and do occur often as a result of some twisting action in the knee or as a result of repetitive impact over time. Similar actions in the other joints can result in similar defects and tears in the similar structures present in those joints. Meniscus degeneration can also occur as a result of aging so that soft or hard areas develop in the tissue such that even common activities such as squatting can cause meniscal tears and defects.
Common surgical procedures for treating meniscal damage include repairing the tears and complete or partial meniscectomies. Repairing a tear is commonly performed when the tear is a longitudinal vertical tear in the vascular (or red) zone of the meniscus. The tear walls can be rasped or trephined to induce bleeding, especially when the tear is just beyond the borders of the red zone (i.e. in the red/white zone). The tear is stabilized with suture or some other repair device such that the relative motion of the tear faces is minimized or eliminated during load bearing. Also, the knee capsule tissue (i.e. synovium) is sometimes rasped to induce bleeding of this highly vascularized tissue into the joint with the intent to provide a better healing environment for meniscal tears. Many devices and surgical procedures exist for repairing meniscal tears by approximating the faces of the meniscal tear. Examples of such devices and procedures are disclosed in the following U.S. Pat. Nos. 6,319,271; 6,306,159; 6,306,156; 6,293,961; 6,156,044; 6,152,935; 6,056,778; 5,993,475; 5,980,524; 5,702,462; 5,569,252; 5,374,268; 5,320,633; and 4,873,976. The other common meniscal procedure, meniscectomy, involves the surgical removal of part of or all of the meniscus. Such procedures have commonly been performed in the case of “un-repairable” or complex tears such as radial tears, horizontal tears, vertical longitudinal tears outside the vascular zone, defibrillation, and/or degeneration. Defects that occur in the avascular (white) or limited vascular (red/white) areas typically do not heal. Meniscectomies typically provide immediate pain relief and restoration of knee function to the patient; however, with the absence of the meniscus, the long term effect on the knee can be cartilage wear on the condylar or tibial plateau surfaces and the eventual development of an arthritic condition such as osteoarthritis. Osteoarthritis is a result of cartilage degradation that is associated with chronic knee pain and often leads to total joint reconstruction. It is for these reasons that meniscal scaffolds and implants have been developed to regenerate or replace the tissue that is removed during a partial or total meniscectomy (see, for instance, U.S. Pat. Nos. 6,042,610; 5,735,903; 5,681,353; 5,108,438; 5,007,934; and 4,880,429).
Clinical experience indicates that white zone and red/white zone tears and defects typically do not heal even if they are stabilized with standard repair techniques. The option of not treating these types of defects is known to result in propagation of tears and defects and degeneration of the meniscus and subsequent degeneration of the articular cartilage and development of osteoarthritis. However, studies performed by Dr. Steven Arnoczky in animals [Arnoczky S P, Warren R F, Spivak J M; J Bone Joint Surg Am. 1988 Sep; 70(8):1209-17, “Meniscal repair using an exogenous fibrin clot. An experimental study in dogs.”] and human clinical experience has shown that if the white or red/white zone defect surfaces are in contact with a blood clot (i.e. fibrin clot) then such tears or defects have a greater propensity to heal. So, if a surgeon were to deliver and fix a blood or fibrin clot to tear or defect surfaces, then healing would likely occur. Most surgeons, however, do not attempt to deliver and fix blood/fibrin clots to facilitate the repair of these types of tears because of the technical challenges. These meniscal procedures are typically performed using arthroscopic techniques (i.e. through small portals using an arthroscope or camera to visualize the surgical site). In order to see clearly through the arthroscope, the surgeon is required to constantly infuse the knee with fluid (i.e. saline solution, Ringer's solution, etc. . . . ); however, if he or she is trying to deliver a blood clot and fix it in the white or red/white zone defect, then the fluid would typically be turned off so that the clot does not disintegrate during the delivery and fixation stage. With the fluid turned off, the surgeon has the technical challenge of not being able to see the surgical site clearly; therefore, a technical dilemma exists: in order to see more clearly the fluid needs to be turned on, but in order to deliver and fix the clot the fluid needs to be turned off. Therefore, the technical challenges are too difficult to overcome in an arthroscopic environment; the surgeon therefore typically excises injured or degenerated white zone and red/white zone tissue (i.e. performs a partial or total meniscectomy). Performing these procedures in a non-arthroscopic setting (i.e. open condition) is not a viable option due to patient expectations, increased morbidity, and increased risks associated with larger incisions.
Currently, tissue engineering scaffolds are being developed to replace the meniscal tissue that has been removed, such as for instance, (ReGen Biologics' Collagen Meniscal Implant or CMI and DePuy's (a Johnson & Johnson company) small intestine submucosa meniscal implant. These implants are being developed to regenerate meniscal tissue; however, they are effective only when the implant is placed in direct contact with the vascular (red) zone of the meniscus. Therefore, if the defect area is confined to the avascular zone only, then one of the meniscal implants referred to above will not regenerate that tissue. For the defects that are confined to the avascular zone only, the surgeon must then remove only that portion of the meniscus that is injured and/or diseased and would not expand the defect into the vascular zone, thus removing “good tissue.” So, for those patients with avascular zone defects, the only option today (and even in the future with the above mentioned tissue engineered scaffolds in their current configuration) is a partial meniscectomy with no tissue engineering replacement solution. Unfortunately for the patient who receives the partial meniscectomy, the long term prognosis includes chronic knee pain, break down of the articular cartilage, osteoarthritis, and even eventual total knee replacement.
Similar to the knee meniscus, other structures are found throughout the body that have avascular and vascular anatomies in close proximity where the avascular portion of these structures have very little propensity for healing. Some of these other structures are the labrum of the hip joint, the labrum of the shoulder joint, the meniscal-like structure of the wrist, the discs of the spine, the disc of the temporomandibular joint, diseased cardiac muscle (i.e. due to reduced blood flow from cardiovascular blockage) to name a few.
Also, in a spinal application, when a patient presents to a surgeon with a bulging or herniated or ruptured spinal disc, the adjacent vertebral bone is often sclerotic (i.e. thickened or denser). Since much of the nutrients for the spinal disc are delivered via diffusion through the vertebral endplates, the sclerotic bone could tend to decrease the amount of nutrients delivered to the disc, thus contributing to the diseased state of the disc.